In the future Internet it is likely that diverse service requirements will create a strong demand for the ability to modify a network subsystem's functionality, if possible at runtime and in response to customisation needs. To date, proposals for dynamic tuning of functionality have used various ad hoc techniques for cross-layer optimisation. Several frameworks have also been proposed by researchers from the active networking community; these enable incremental extension of functionality. We believe that past work has only partially addressed the goal of functional composition. In this article we propose an integrated system that is capable of functional adaptation in response to a variety of adaptation objectives. We describe a prototype implementation of our system and an evaluation of the prototype.